The present invention pertains generally to a step tablet (step wedge), which is a device employed for testing an amount of exposure of a photographic plate or the degree of exhaustion of a developing solution used for developing a photographing plate. Such devices find use, for instance, in determining the amount of exposure of a photographic lithographic printing plate or the like. Particularly, the invention pertains to a step tablet that enables a more certain and easier determination to be made as to the amount exposure or degree of exhaustion of a developing solution.
Conventionally, a step tablet has been used to determine the proper exposure and development conditions for positive presensitized (PS) plates. In use, the step tablet is placed adjacent the photosensitive surface of the plate, and then the plate is exposed through the step tablet. Upon developing, the plate should have discernible several shades of gray, about five in the typical application.
A conventional step tablet is illustrated in FIG. 1 of the drawings. The tablet is composed of a number of patches or steps having different optical densities. In the example here given, the density varies from a minimum value of 0.05 in increments of 0.15. Each step has a rectangular shape.
Each of the steps is marked with an Arabic numeral. The optical density of all of the Arabic numerals corresponds to the minimum optical density among the various steps. Hence, because the step of minimum density and the numeral which marks it have the same density, that numeral is indiscernible. The step having the most clearly discernible numeral is the step of maximum density, here marked by "15".
After developing the plate, the operator examines the area of the plate which was exposed through the step tablet to determine the last step, in descending order of optical density, having a discernible marking numeral. Above that step will be found approximately five steps of varying optical density. An example of this area of a developed plate is shown in FIG. 2.
It is sometimes quite difficult to accurately determine the last step for which the numeral is visible. For instance, it might not be readily possible to determine whether the steps marked with numerals "15" to "10" are all solid, or whether the numerals "15" to "9" are all solid, that is, it might not be possible to determine whether the step marked "10" or the step marked "9" is the last step having a discernible numeral.
Accordingly, it is an object of the present invention to provide a step tablet in which the amount of exposure or the quality of a developing solution can be readily and accurately determined.
More specifically, it is an object of the present invention to provide a step tablet with which the amount of exposure or the quality of a developing solution can readily be ascertained by quickly identifying a particular step on the step tablet.